Effect of preheating of the reactants on the transition to thermoacoustic instability in a bluff-body stabilized dump combustor

2020 ◽  
Author(s):  
Samadhan A. Pawar ◽  
Manikandan Raghunathan ◽  
K.V. Reeja ◽  
P.R. Midhun ◽  
R.I. Sujith
Keyword(s):  
Bluff Body ◽  
Thermoacoustic Instability ◽  
Dump Combustor

Smart Passive Control of Thermoacoustic Instability in a Bluff-Body Stabilized Combustor: A Lagrangian Analysis of Critical Structures

2021 ◽  
Author(s):  
C P Premchand ◽  
Manikandan Raghunathan ◽  
Midhun Raghunath ◽  
K V Reeja ◽  
Sujith R I ◽  
...  
Keyword(s):  
Bluff Body ◽  
Passive Control ◽  
Lagrangian Analysis ◽  
Thermoacoustic Instability

Smart Passive Control of Thermoacoustic Instability in a Bluff-Body Stabilized Combustor: A Lagrangian Analysis of Critical Structures

2020 ◽  
Author(s):  
C. P. Premchand ◽  
Manikandan Raghunathan ◽  
Midhun Raghunath ◽  
K. V. Reeja ◽  
R. I. Sujith ◽  
...  
Keyword(s):  
Wavelet Transform ◽  
Flow Field ◽  
Heat Release ◽  
Shear Layer ◽  
Heat Release Rate ◽  
Release Rate ◽  
Sound Production ◽  
Bluff Body ◽  
Passive Control ◽  
Thermoacoustic Instability

Abstract The tonal sound production during thermoacoustic instability is detrimental to the components of gas turbine and rocket engines. Identifying the root cause and controlling this oscillatory instability would enable manufacturers to save in costs of power outages and maintenance. An optimal method is to identify the structures in the flow-field that are critical to tonal sound production and perform control measures to disrupt those “critical structures”. Passive control experiments were performed by injecting a secondary micro-jet of air onto the identified regions with critical structures in the flow-field of a bluff-body stabilized, dump, turbulent combustor. Simultaneous measurements such as unsteady pressure, velocity, local and global heat release rate fluctuations are acquired in the regime of thermoacoustic instability before and after control action. The tonal sound production in this combustor is accompanied by a periodic flapping of the shear layer present in the region between the dump plane (backward-facing step) and the leading edge of the bluff-body. We obtain the trajectory of Lagrangian saddle points that dictate the flow and flame dynamics in the shear layer during thermoacoustic instability accurately by computing Lagrangian Coherent Structures. Upon injecting a secondary micro-jet with a mass flow rate of only 4% of the primary flow, nearly 90% suppression in the amplitude of pressure fluctuations are observed. The suppression thus results in sound pressure levels comparable to those obtained during stable operation of the combustor. Using Morlet wavelet transform, we see that the coherence in the dominant frequency of pressure and heat release rate oscillations during thermoacoustic instability is affected by secondary injection. The disruption of saddle point trajectories breaks the positive feedback loop between pressure and heat release rate fluctuations resulting in the observed break of coherence. Wavelet transform of global heat release rate shows a redistribution of energy content from the dominant instability frequency (acoustic time scale) to other time scales.

Smart Passive Control of Thermoacoustic Instability in a Bluff-Body Stabilized Combustor: A Lagrangian Analysis of Critical Structures

2021 ◽  
Author(s):  
C P Premchand ◽  
Manikandan Raghunathan ◽  
Midhun Raghunath ◽  
K V Reeja ◽  
Sujith R I ◽  
...  
Keyword(s):  
Bluff Body ◽  
Passive Control ◽  
Lagrangian Analysis ◽  
Thermoacoustic Instability

Lean Blowout Phenomena and Prior Detection of Lean Blowout in a Premixed Model Annular Combustor

2019 ◽  
Author(s):  
Arijit Bhattacharya ◽  
Bikash Gupta ◽  
Satyajit Hansda ◽  
Zohadul Haque ◽  
Ashish Kumar ◽  
...  
Keyword(s):  
Bluff Body ◽  
Nox Reduction ◽  
Marked Change ◽  
Nox Emission ◽  
Thermoacoustic Instability ◽  
Lean Blowout ◽  
Lean Combustion ◽  
Annular Combustor ◽  
Lean Limit ◽  
Lift Off

Abstract Strict emission norms in the last few decades have paved the path for adaptation of new low NoX emission alternatives to power generation and aircraft propulsion. Lean combustion is a very promising and practicable technology for reducing NOX reduction and also have very high fuel efficiency. However, lean combustion technology suffers from inherent combustion instabilities that are manifested under different conditions, most importantly, thermoacoustic instability and lean blowout. Lean blowout occurs when a gas turbine combustor operating close to lean limit, for lowest NoX emission, faces abrupt changes in fuel homogeneity, quality or flow rate. While many work have been done in thermo-acoustic instability and flame propagation in annular combustors, studies in lean blowout in annular combustors are very limited. The lean limit of combustors are not fixed and is dependent on fuel characteristics and operating condition including environmental effects. So accurate online prediction of lean limit is very important to keep the combustors operating safely near lean limit. Recent works have demonstrated that single burner combustors leave out a significant amounts of physics including interaction of flames from different burners prior to blowout. In this work, a stepped down swirl and bluff body stabilized annular combustor in CB configuration (having chamber and burner), is used as experimental test rig having 4 number of identical burners. Video and heat release data are taken at different conditions as lean blowout is approached. Frequent attachment and reattachment of the flames prior to lift off was seen. As lean blowout is approached, inherent subtle differences in the different burners get amplified when flame becomes sufficiently weak and flame symmetry is broken. As air fuel mixture is made gradually leaner, one by one the flames from different burners elongates although remains partially attached to burner. Further lowering the equivalence ratio results in lift off and merging of the flame fronts of different burners. Three pixel averaged color ratios are extracted from still camera RGB images as flame stability indicators which are, red by blue, red by green and blue by green. The parameters show marked change at the point of lift off as well as at the lean blowout point.

Lagrangian Saddle Point Analysis in the Flow-Field of a Bluff-Body Stabilized Combustor

2019 ◽  
Author(s):  
C. P. Premchand ◽  
Nitin B. George ◽  
Manikandan Raghunathan ◽  
Vishnu R. Unni ◽  
R. I. Sujith ◽  
...  
Keyword(s):  
Saddle Point ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Bluff Body ◽  
Leading Edge ◽  
Mean Flow ◽  
Acoustic Frequency ◽  
Thermoacoustic Instability ◽  
Point Analysis

Abstract Experiments are performed in a partially-premixed bluff-body stabilized turbulent combustor by varying the mean flow velocity. Simultaneous measurements obtained for unsteady pressure, velocity and heat release rate are used to investigate the dynamic regimes of intermittency (10.1 m/s) and thermoacoustic instability (12.3 m/s). Using wavelet analysis, we show that during intermittency, modulation of heat release rate occurring at the acoustic frequency fa by the heat release rate occurring at the hydrodynamic frequency fh results in epochs of heat release rate fluctuations where the heat release is phase locked with the acoustic pressure. We also show that the flame position during intermittency and thermoacoustic instability are essentially dictated by saddle point dynamics in the dump plane and the leading edge of the bluff-body.

Lagrangian Analysis of Flame Dynamics in the Flow Field of a Bluff Body-Stabilized Combustor

2019 ◽  
Vol 142 (1) ◽  
Author(s):  
C. P. Premchand ◽  
Nitin B. George ◽  
Manikandan Raghunathan ◽  
Vishnu R. Unni ◽  
R. I. Sujith ◽  
...  
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Bluff Body ◽  
Leading Edge ◽  
Mean Flow ◽  
Lagrangian Analysis ◽  
Acoustic Frequency ◽  
Thermoacoustic Instability ◽  
Flame Dynamics

Abstract Experiments are performed in a partially premixed bluff body-stabilized turbulent combustor by varying the mean flow velocity. Simultaneous measurements obtained for unsteady pressure, velocity, and heat release rate are used to investigate the dynamic regimes of intermittency (10.1 m/s) and thermoacoustic instability (12.3 m/s). Using wavelet analysis, we show that during intermittency, modulation of heat release rate occurring at the acoustic frequency fa by the heat release rate occurring at the hydrodynamic frequency fh results in epochs of heat release rate fluctuations where the heat release rate is phase locked with the acoustic pressure. We also show that the flame position during intermittency and thermoacoustic instability are essentially dictated by saddle point dynamics in the dump plane and the leading edge of the bluff body.

Heat Release Response to Forced Flow Oscillations of a Low-Order Modelled Laboratory Scale Dump Combustor

2015 ◽  
Author(s):  
Alessandro Orchini ◽  
Matthew P. Juniper
Keyword(s):  
Flow Field ◽  
Heat Release ◽  
Bluff Body ◽  
Forced Flow ◽  
Mean Flow ◽  
Order Model ◽  
Qualitative Comparison ◽  
Velocity Fluctuations ◽  
Dump Combustor ◽  
Low Order

In this study we investigate the heat release response to forced harmonic velocity fluctuations of a bluff body dump combustor. We use the kinematic G-equation as a low-order model for the flame and heat release dynamics. The geometry considered is based on an experimental setup developed by R. Balachandran and widely investigated in the literature, and we look for a qualitative comparison with experimental and numerical results. We model the flow field dynamics adapting the well-known travelling wave model, which was originally developed for conical flames, to the case of a bluff-body stabilized flame in a non-uniform mean flow field. We impose velocity fluctuations at the dump plane at various frequencies and amplitudes and we integrate the nonlinear flame and heat release dynamics. Results show that the model qualitatively reproduces the kinematic behaviour observed in the experiments, although some major quantitative differences are found. We conclude by discussing our results, and adjustments that could be introduced in future in the low-order model in order to improve it.

Intermittency route to thermoacoustic instability in turbulent combustors

2014 ◽  
Vol 756 ◽  
pp. 470-487 ◽  
Author(s):  
Vineeth Nair ◽  
Gireeshkumaran Thampi ◽  
R. I. Sujith
Keyword(s):  
Bluff Body ◽  
Combustion Instability ◽  
Signal Amplitude ◽  
High Amplitude ◽  
Operating Conditions ◽  
Combustion Noise ◽  
Continuous Measure ◽  
Periodic Oscillations ◽  
Thermoacoustic Instability ◽  
Amplitude Fluctuations

AbstractThe dynamic transition from combustion noise to combustion instability was investigated experimentally in two laboratory-scale turbulent combustors (namely, swirl-stabilized and bluff-body-stabilized backward-facing-step combustors) by systematically varying the flow Reynolds number. We observe that the onset of combustion-driven oscillations is always presaged by intermittent bursts of high-amplitude periodic oscillations that appear in a near-random fashion amidst regions of aperiodic low-amplitude fluctuations. These excursions to periodic oscillations last longer in time as operating conditions approach instability and finally the system transitions completely into periodic oscillations. A continuous measure to quantify this bifurcation in dynamics can be obtained by defining an order parameter as the probability of the signal amplitude exceeding a predefined threshold. A hysteresis zone was observed in the bluff-body-stabilized configuration that was absent in the swirl-stabilized configuration. The recurrence properties of the dynamics of intermittent burst oscillations were quantified using recurrence plots and the distribution of the aperiodic phases was examined. From the statistics of these aperiodic phases, robust early-warning signals of an impending combustion instability may be obtained.

Thermoacoustic instability as mutual synchronization between the acoustic field of the confinement and turbulent reactive flow

2017 ◽  
Vol 827 ◽  
pp. 664-693 ◽  
Author(s):  
Samadhan A. Pawar ◽  
Akshay Seshadri ◽  
Vishnu R. Unni ◽  
R. I. Sujith
Keyword(s):  
Heat Release ◽  
Limit Cycle ◽  
Acoustic Field ◽  
Heat Release Rate ◽  
Release Rate ◽  
Bluff Body ◽  
Acoustic Pressure ◽  
Mutual Synchronization ◽  
Periodic Oscillations ◽  
Thermoacoustic Instability

Thermoacoustic instability is the result of a positive coupling between the acoustic field in the duct and the heat release rate fluctuations from the flame. Recently, in several turbulent combustors, it has been observed that the onset of thermoacoustic instability is preceded by intermittent oscillations, which consist of bursts of periodic oscillations amidst regions of aperiodic oscillations. Quantitative analysis of the intermittency route to thermoacoustic instability has been performed hitherto using the pressure oscillations alone. We perform experiments on a laboratory-scale bluff-body-stabilized turbulent combustor with a backward-facing step at the inlet to obtain simultaneous data of acoustic pressure and heat release rate fluctuations. With this, we show that the onset of thermoacoustic instability is a phenomenon of mutual synchronization between the acoustic pressure and the heat release rate signals, thus emphasizing the importance of the coupling between these non-identical oscillators. We demonstrate that the stable operation corresponds to desynchronized aperiodic oscillations, which, with an increase in the mean velocity of the flow, transition to synchronized periodic oscillations. In between these states, there exists a state of intermittent phase synchronized oscillations, wherein the two oscillators are synchronized during the periodic epochs and desynchronized during the aperiodic epochs of their oscillations. Furthermore, we discover two different types of limit cycle oscillations in our system. We notice a significant increase in the linear correlation between the acoustic pressure and the heat release rate oscillations during the transition from a lower-amplitude limit cycle to a higher-amplitude limit cycle. Further, we present a phenomenological model that qualitatively captures all of the dynamical states of synchronization observed in the experiment. Our analysis shows that the times at which vortices that are shed from the inlet step reach the bluff body play a dominant role in determining the behaviour of the limit cycle oscillations.

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